Storage area networks, also referred to as SANs, are dedicated networks that connect one or more systems to storage devices and subsystems. Today, fibre channel is one of the leading technologies for SANs. In general, fibre channel encompasses three networking topologies: point-to-point, loop, and fabric. In a point-to-point topology, a fibre channel host adapter in a system is connected to a single fibre channel storage subsystem. In a fibre channel loop network, also called an arbitrated loop, the loop is constructed by connecting nodes together in a single logical ring. Loops can be constructed by connecting nodes through a fibre channel hub in a star-wired topology or by connecting them together in a connected physical loop from node to node. In a fibre channel fabric topology, the storage networks are constructed with network switches. A fabric can be composed of a single switch or multiple switches. Ports on fabric networks connect nodes to switches on low-latency, point-to-point connections.
In fibre channel loop topologies, diagnostics often becomes difficult because the problems are often propagated across the entire loop. One method of isolating faulty devices in a loop is by a process of elimination. This consists of running a series of link tests on the loop by bypassing individual devices in the loop (i.e., replacing/removing components on the loop) until one or more faulty links are identified. This method results in excessive MTTD (mean time to diagnose) and may require field personnel to be present. Furthermore, this method requires additional hardware and bypass circuitry to be added to the devices connected to the loop.